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Biological samples of Isotope concentrations of Cesium 134 and 137, Silver 110m, and Potassium 40 from cruise KOK1108 in June 2011 in the Western equatorial Pacific and Kurushio Extension (Fukushima Radionuclide Levels project)

Counts and concentrations of Cs134, Cs137, Ag110m, and K40 detected in zooplankton and small fish samples are reported. Samples were collected as part of a radioecological study of biota in order to assess the impact of radiation leaks from the Fukushima Daiichi nuclear power facility, damaged by a March 11, 2011 earthquake and tsunami. Radionuclide results were determined from high purity germanium detectors and calibrated against IAEA standards as described in Buesseler et al. (PNAS, 2012).

Mixed zooplankton samples were sampled using Bongo nets (mesh size = 300 micromol and Methot net (mesh size = 4mm) to collect gelatinous zooplankton, larger crustaceans and fish. Samples collected were pooled together from several casts to achieve biomass needed for radioanalysis. Samples were frozen prior to freeze-drying. Samples that were freeze-dried and ground were stored and analyzed in straight side Nalgene 4oz jar; powdered mass was compressed by a polyacrilamide ring placed on top to assure uniform distribution of the sample in the jar. Biological samples were analyzed using a planar low energy germanium detector - LEGe, Canberra, Model GLP 3830 with a 3800 mm2 active area; Genie 2000 software was used for spectrum analysis.

Processing Description

Total counting efficiencies that accounted for sample geometry and for energy emission were calculated based on standards composed of Se-75, Cs-137 and Eu-152 (265 keV, 662 keV and 1408 keV, respectively). We also accounted for the branching ratio and the concentrations were decay-corrected back to April 6th, 2011 The error reported reflects the % counting error. The lower end of the detection was decided based on the lowest among the samples radionuclide concentration minus one error. We did not correct for coincidence summing. An algorithm used for making these corrections - i.e. total % counting efficiency was: CEtot% = ((-6.24 * ln(sample volume) + 48.23) * (13.74 * energy * ^(-0.79)))/100;

A Bongo Net consists of paired plankton nets, typically with a 60 cm diameter mouth opening and varying mesh sizes, 10 to 1000 micron. The Bongo Frame was designed by the National Marine Fisheries Service for use in the MARMAP program. It consists of two cylindrical collars connected with a yoke so that replicate samples are collected at the same time. Variations in models are designed for either vertical hauls (OI-2500 = NMFS Pairovet-Style, MARMAP Bongo, CalVET) or both oblique and vertical hauls (Aquatic Research). The OI-1200 has an opening and closing mechanism that allows discrete "known-depth" sampling. This model is large enough to filter water at the rate of 47.5 m3/minute when towing at a speed of two knots. More information: Ocean Instruments, Aquatic Research, Sea-Gear

A Methot Net, a type of plankton net, is used to sample juvenile fish, shrimp, and 'larger' plankton, e.g. 4 millimeters and larger. Named after its designer, Richard D. Methot, of La Jolla, California, it is also called a Methot Trawl. It is a single net with a large square opening or mouth. The net is deployed from the stern and towed behind the vessel. The Methot uses fine mesh (e.g. 4 mm) but with openings slightly larger than other plankton net systems. The larger mesh size allows the net to be towed at higher speeds. A flowmeter suspended in the mouth of net measures the flow of water moving through the net and allows for the calculation of the volume of water sampled. With its larger mouth and faster speed through the water, the Methot is designed to catch the larger zooplankton that are often missed by other plankton net samplers.